The Pore Water Pressure and Settlement Characteristics of Soil Improved by Combined Vacuum and Surcharge Preloading

Jie Peng, Wen Guang Ji, Neng Li, Hao Ran Jin 1. Key Laboratory for Ministry of Education for Geo-mechanics and Embankment Engineering, Hohai University, Nanjing, 210098, China 2. Geotechnical Research Institute, Hohai University, Nanjing 210098, China

ABSTRACT This study examines the pore water pressure and settlement characteristics of soil improved by combined vacuum and surcharge preloading based on two field tests. It discusses and compares methods of computing settlement and the degree of consolidation between combined vacuum and surcharge preloading and surcharge preloading alone as well. The non- uniform change of in the underground pore water pressure and the change in water depth indicates that the directly effective range of vacuum pumping in this paper should reach 18 m below the surface and that the range of the decline in the pore water pressure during vacuum preloading increases with decreasing depth below the surface. The groundwater table level declines during the process of vacuum preloading, and the restoration of the negative pore water pressure following unloading requires a period of time, as with the dispersion of the positive excess pore water pressure under surcharge preloading. The combination of vacuum and surcharge load can increase both the rate of the soil settlement and the total soil settlement; however, the settlement increment caused by the vacuum load will be less than that caused by the real surcharge load, and the surface settlement of combined vacuum and surcharge preloading is more uniform than that of surcharge preloading. The vacuum load can be equivalent to the positive load when settlement and the degree of consolidation of the combined vacuum and surcharge preloading are calculated and when the settlement correction coefficient (ms) is less than that of the surcharge preloading. KEYWORDS: vacuum combined surcharge preloading, surcharge preloading, field test, pore pressure, settlement

INTRODUCTION Recently, the construction of highway embankments over soft clayey deposits has resulted in the advancement of soil improvement techniques. Surcharge preloading is a popular and well- developed method used in practical engineering to improve the shear strength of soft soil and to reduce its post-construction settlement. Surcharge preloading generates positive excess pore water pressure in the soil through applying embankments on the ground. Because of the strong - 1559 -

Vol. 18 [2013], Bund. G 1560 permeability of the prefabricated vertical drains(PVDs) inserted into the ground , the pore water pressure in the PVDs remains constant, minimizing the pressure differences produced between the soil mass and the PVDs; this effect causes the pore water to discharge from the soil mass and increases the rate of soil consolidation, thereby reinforcing the soil. Surcharge preloading is widely used in soft soil improvement (1-4). The primary disadvantages of surcharge preloading are that it requires a long preloading time and large quantity of embankment material and that it has an accompanying instability problem. Surcharge preloading can be combined with vacuum preloading to reduce the quantity of fill material required, to accelerate the rate of consolidation, to shorten construction periods and to decrease the problem of embankment instability. Vacuum preloading, originally introduced by W. Kjellman(5), decreases the pressures below the sealing membrane and within the PVDs caused by vacuum pumping. Because of the weak permeability of the soil, the rate of decrease in pore water pressure in the soil is slower than that in the PVDs, and differences in pressure developed between the soil mass and the PVDs. The pressure differentials drive the pore water from the soil to the PVDs, leading to a decrease in the underground pore water pressure, while the total stress is maintained at the same level. As a result, the effective stress of the soil is increased to accelerate consolidation (6). Following this principle, many scholars have studied the consolidation mechanism, calculation methods and construction technologies of vacuum preloading (7-13). The effectiveness of combined vacuum and surcharge preloading has been discussed by Chai et al. (2) and Indraratna et al. (14). In this method, the vacuum pressure can be distributed to a greater depth in the subsoil, and the consolidation time of stage construction can be minimized (10). Moreover, the rate of embankment construction can be increased (15). The post-construction settlement will be significantly less with increasing soil stiffness and shear strength owing to consolidation, thereby eliminating the risk of differential soil settlement (16). Previous research regarding the soil pore water pressure characteristics of soil improved by combined vacuum and surcharge preloading is still insufficient, and comparative field test studies of the deformation characteristics between combined vacuum and surcharge preloading and surcharge preloading alone are rare. With two field tests, this paper examines the pore water pressure and soil settlement characteristics of soil improved by combined vacuum and surcharge preloading in similar geological sites; this paper also discusses and compares the methods of computing settlement and the degree of consolidation for these two methods.

K32+475

1 3.50

23.50 3 27.50

1 Loam 2 M uddy loam 3 Loam 4 Pebbly clay

Figure 1: Geological section map of Test A Vol. 18 [2013], Bund. G 1561

K23+880 K23+720 K23+655 K23+597 0.00 1 1 1 3.10 1 2 3.70 2 4.00 5.50 5.70 5.30

3 3 3 14.80 14.60 3 4 4 19.30 18.50 21.20 6 5 4 23.00 5 23.30 24.60 5 26.00 26.10 7 27.30 7 7 30.10 30.60 6 33.78 32.80 8 8 8 8

1 Cultivated soil 2 Shell bed 3 Muddy clay 4 Silt 5 Silt

6 Clay 7 Muddy Clay 8 Pebble

Figure 2: Geological section map of Test B

OVERVIEW OF FIELD TESTS The two field tests discussed in this paper are conducted in Hangzhou city, Zhejiang province, China (hereinafter referred to as Test A) and in Jiangmen city, Guangdong province, China (hereinafter referred to as Test B).

Project profile of Test A Hangzhou-Jinhua-Quzhou Expressway is an important artery in the expressway network of Zhejiang province in China. Soft soil is common in Hangzhou city, the provincial capital of Zhejiang. In this area, the embankment is high and deep, beneath which soft soil is located; therefore, the combined vacuum and surcharge preloading method is used to treat the soft soil in this area. The section of Test A examined in this paper was named as the VS-A-1 section. The geological section in this section and the properties of each soil layer are illustrated in Figure 1 and Table 1. A portion of the construction parameters in this section is listed in Table 2. The loading curves are shown in Figure 3. The monitoring instruments and their positions in this cross section can be observed in Figure 5. Vol. 18 [2013], Bund. G 1562

160

140

120

100 Surcharge Load 80 Vacuum Load

60 Load(kPa)

0 0 100 200 300 400 500 600 700 Time(days)

Figure 3: Loading curves of test A

200 180 160 140 120 100 Surcharge Load(VS-B-2) 80 Surcharge Load(VS-B-1) Load(kPa) 60 Surcharge Load(S-B-2) 40 Surcharge Load(S-B-1) Vacuum Load(VS-B-2) 20 Vacuum Load(VS-B-1) 0 0 100 200 300 400 500 600 700 Time(days)

Figure 4: Loading curves of test B

Project profile of Test B Zhongshan-Jiangmen Expressway is an important part of the expressway network of Guangdong province in China; the soft soil in the section K23+565.3-K23+987.8 is thick (approximately 21-31 m in thickness), and the embankment fill is higher (6.7-7.8 m in fill height). In this area, surcharge preloading and combined vacuum and surcharge preloading are both adopted to treat the soft ground. The geological section and soil layer distribution for each cross-section are described in Figure 2. The basic parameters in each soil layer are shown in the Table 1. The treatment range, construction parameters in the corresponding parts and the sections studied with these two Vol. 18 [2013], Bund. G 1563

methods are listed in Table 2. For the sake of simplicity and clarity, the surcharge preloading sections, K23 + 597 and K23 + 655, are hereafter referred to as S-B-1 and S-B-2, respectively, and the combined vacuum and surcharge preloading sections, K23 + 720 and K23 + 880, are hereafter referred to as VS-B-1 and VS-B-2, respectively.

Table 1: Geological parameters of two field tests (average value) Water Unit Liquid Plastic -3 Field content weight Void limit limit av Cv (×10 Soil layer  -1 2 test ω 0 ratio e0 WL WP (Mpa ) cm /s) (%) (kN/m3) (%) (%) 1. lloam 28.5 17.5 0.796 20.4 34.3 0.370 5.20 A 2. muddy loam 40.0 18.1 1.040 30.7 43.2 0.671 2.50 3. loam 32.5 18.0 0.902 21.9 35.1 0.342 3.20 4. pebbly clay 18.5 17.8 0.589 - - 0.180 4.50 1. cultivated soil 39.7 18.0 1.056 - - 0.350 8.60 2. shell/oyster 38.9 18.4 1.001 - - 0.150 30.40 3. muddy clay 54.3 18.0 1.271 23.6 38.9 0.921 2.07 B 4. silt 56.6 18.2 1.280 24.9 45.2 0.621 2.90 5. silt 45.2 18.2 1.056 27.4 41.4 0.150 40.00 6. clay 57.7 18.5 1.172 29.2 42.4 0.560 3.50 7. muddy clay 46.2 17.8 1.623 28.2 47.6 1.570 1.35 8. pebble/gravel 33.4 18.6 0.901 - - 0.100 386.00

subgrade centerline settlement plate

settlement plate settlement plate

14m 28m 14m

Figure 5: Settlement monitoring points for test A

Vol. 18 [2013], Bund. G 1564

28m Sf4L Sf4C Sf4R

earth pressure cell 50cm gravel underlayment

Se4 U4C2.0 U4R2.0 U4C4.0 U4C6.0 U4R6.0 U4C8.0 U4C10.0 U4R10.0 U4C14.0 U4R14.0

U4C18.0 U4R20.0 SL21.0m Sh23.0m Sd23.0m Figure 6: Settlement monitoring points for test B

Table 2: Construction parameters of soft soil PVD parameter Thicknes s of sand Filling Section Treatment method Spacing Length cushion height (m) (m) (m) (m) VS-A-1(K32+475) vacuum combined surcharge 1.2 22 0.6 5.9

S-B-1(K23+597) surcharge 1.2 21 0.6 6.7

S-B-2(K23+655) surcharge 1.3 21 0.6 7.1

VS-B-1(K23+720) vacuum combined surcharge 1.3 31 0.8 8.1

VS-B-2(K23+880) vacuum combined surcharge 1.3 21 0.8 8.6

Figure 4 shows the loading curves of each section. The vacuum loads in sections VS-B-1 and VS-B-2 are essentially consistent, differing slightly in the surcharge partitioning. The surcharge speed in the combined vacuum and surcharge preloading area is higher than that in the surcharge preloading area. The section size and the settlement monitoring positions are shown in Figure 6.

TEST RESULTS

The pore water pressure characteristics The pore water pressure analysis is based on the monitoring data from test A. The pore water pressure curves for each depth of test A are illustrated in Figures 7 and 8. The soil pressure Vol. 18 [2013], Bund. G 1565 change can be divided into four stages: (1) The vacuum preloading stage, in which the pore water pressure at each depth decreases; (2) The surcharge preloading stage, in which the pressure increases gradually with the application of the surcharge load; (3) The combined preloading stage, after the completion of surcharge the pressure dissipates gradually and decreases; 4) The post vacuum unloading stage, in which the pressure in the soil increases and later decreases slightly until it becomes stable. The surcharge load begins on the 65th day after vacuum preloading. Before the application of surcharge, the reinforced area is in the vacuum preloading stage. Analyses are completed on the pore water pressures at different stages to further understand the change of pore water pressure in the vacuum preloading stage. 250

200 U4C18.0 U4C14.0 150 U4C10.0 U4C8.0 100 U4C6.0 U4C4.0 U4C2.0 50

Pore water pressure(kPa) 0 0 100 200 300 400 500 600 700 800 Time(days) -50 Figure 7: Measured pore water pressure values at the center of the section

250

200 U4R20.0 150 U4R14.0 U4R10.0 U4R6.0 100 U4R2.0

Pore water pressure(kPa) 0 0 100 200 300 400 500 600 700 8 Time(day) -50 Figure 8: Measured pore water pressure values on the right side of the section Vol. 18 [2013], Bund. G 1566

Pore water pressure increment(kPa) -50 -40 -30 -20 -10 0 10 0 2 4 6 8 10

15th day 12 Depth(m) Depth(m) 26th day 14 40th day 46th day 16 18 20

Figure 9: Pore water pressure for different time periods at the embankment center

Time( days) 0 5 10 15 20 25 30 35 0

-10

-20 ges at different depths different at ges 2m -30 6m 10m -40 14m 18m

-50 Pore water pressure chan

Figure 10: Pore water pressure at different depths at the embankment center

Pore water pressure increment(kPa) -80 -60 -40 -20 0 20 0 2 4 6 15th day 8 26th day 10 40th day 12 46th day Depth(m) 14 16 18 20 Figure 11: Pore water pressure for different periods on the right side of the embankment Vol. 18 [2013], Bund. G 1567

Time(day) 0 5 10 15 20 25 30 0

-5

-10

-15

-20

-25 2m -30 6m -35 10m 14m -40 18m -45

Pore water pressure changes at different depths(kPa) at different changes pressure water Pore Figure 12: Pore water pressure at different depths on the right side of embankment

Pore water pressure increment (kPa) -60-50-40-30-20-100 10 0 2 4 6 8 10 12 46th day(right side) Depth(m) 15th day(right side) 14 46th day(center) 15th day(center) 16 18 20

Figure 13: Pore water pressure at the center and on the right side of the embankment

Vol. 18 [2013], Bund. G 1568

Pore water pressure increment(kpa) -40-200 20406080 0

4 Pore water pressure increment caused by the interruption 8

Depth(m) Pore water pressure increment before the interruption Pore water pressure increment 12 after the interruption

20 Figure 14: Pore water pressure increments before and after the power interruption

(1) Analysis of the pore water pressure during the vacuum pumping stage The incremental pore water pressure at different time intervals of vacuum preloading is shown in Figures 9 and 10 As shown in Figures 9 and10, the pore water pressure in the soil decreases gradually with the increasing time of the vacuum preloading; that is, the excess pore water pressure is negative, and the absolute value of the negative excess pore water pressure increases with time. The changes in the pore water pressure at different depths at different time intervals of vacuum preloading are illustrated in Figure 11and Figure 12; the range of the decline and the speed of the decline both decrease with increasing soil depth. For instance, on the 46th day of vacuum preloading, the pore water pressure measured at a depth of 2m in these two survey points decrease by 45 and 48 kPa, by 28.5 and 27.2kPa at 10 m, by 15.3 kPa at 18 m beneath the middle survey point, and by 12.1 kPa at 20 m beneath the survey point at the edge. This result suggests that there is a significant well resistance in the PVDs, which affects the transmission of the vacuum. The decline in the pore water pressure on the right side and in the center of the reinforced area from the 15th day to the 46th day after vacuum preloading can be observed in Figure 13. The range in the decline of the pore water pressure in the center and at the edge of the reinforced area are essentially in accordance; that is, the vacuum action is consistent from the center to the edge. The vacuum action in the soil at the same depth is essentially consistent because the vacuum action is primarily transmitted by the PVDs, and the vacuum distribution under the membrane is uniform in general. Thus, the settlement in the reinforced area under vacuum preloading is comparatively uniform. The horizontal settlement tube data are shown in Figure 15. In comparison with the combined vacuum and surcharge preloading stage, the settlement differences between the center and the edge during the vacuum preloading stage are significantly smaller. Vol. 18 [2013], Bund. G 1569

Distance from the center(m) -30 -20 -10 0 10 20 30 0

500 42 days in vacuum Surcharge load = 88kPa Vacuum unloaded 1000

1500

2000

2500 Settlement(mm)

3000 Figure 15: Horizontal settlement tube data for K32+475

Time(days) 0 5 10 15 20 25 30 35 40 45 0

10 Settlement at the center 15 Settlement on the bottom of 20 the plastic drainage plate

30 Settlement(cm) 35

Figure 16: Settlement at the surface and on the bottom of the plastic drainage plate of the reinforced area during the vacuum

(2) Analysis of the ground water level change Two reasons are given for the decline in the underground pore water pressure caused by the vacuum preloading: on the one hand, the vacuum causes the pore water pressure in the PVDs to decline, transmitting the pressure in all directions, and this influence decreases with increasing depth; on the other hand, the change in the ground water level leads to the decline in the pore water pressure, which has a consistent influence at all depths below the water level. There are many disputes regarding the changes in the ground water level during vacuum preloading. In this study, the water level in the reinforced area declines. As shown in Figures 7 and 8, the change in the pore water pressure at a depth of 2 m is different from that at other depths; during the application of the surcharge, the pore water pressure at other depths increases with the increasing load; however, the pressure at 2 m underground remains essentially the same, demonstrating that the soil mass at this depth is unsaturated. If the soil were saturated at the depth of 2 m, it would not be sensitive to the vacuum change yet insensitive to the surcharge stress. The change in the pore water pressure at 4 m is consistent with the surcharge, demonstrating that the ground water level during the vacuum preloading is between 2 m and 4 m. Vol. 18 [2013], Bund. G 1570

The initial water level was at 1.0 m below the soil surface, and the water level floating range outside the reinforced area was within 0.5 m, which indicates that the water level declines caused by the vacuum preloading were greater than 1 m. From the change in the pore water pressure at 18 m, during the 50-day vacuum preloading period, the range of decline in pressure is up to 18 kPa. Without considering the effect of the vacuum on the pore water pressure decline, it is clear that the range of decline in the water level among the PVDs will not exceed 1.8 m; that is, the range of decline in the ground water level is from 1 m to 1.8 m. (3) The change in the pore water pressure after the interruption of vacuum pumping From the 65th to the 66th day of the vacuum pumping, the pumping is interrupted for 2 days due to a power failure. The interruption of the pumping causes an increase in the pore water pressure in the ground, and the changes in the pore water pressure at each depth after the interruption of pumping are shown in Figure 14. The pore water pressure at 0~6 m below the surface greatly increases again, but only a slight increase at 6~18 m underground. Through measurement, the maximum value of the increase is 45.8 kPa at 2 m in depth, and the range of increase in the pore water pressure below 6 m is smaller and similar with an average increase of approximately 5 kPa. The decrease in the underground pore water pressure caused by vacuum pumping is gradual, but the increase is also delayed following the interruption of vacuum pumping. The pore water pressure does not recover from the hydrostatic pressure state immediately when the vacuum pumping interrupted, and the rapid recovery of the pore water pressure at a depth of 2 m is because of its unsaturated state and its hydrodynamic pressure primarily consists of the gas pressure. Therefore, the increase and decrease happen quickly. The figure illustrating the change in the pore water pressure shows that the change in the level of vacuum under the membrane influences the pore water pressure in shallow soil more than that in deeper ground. The increase in the pore water pressure at 18 m underground is less than 5 kPa, indicating that the increase in the ground water level during the interruption of pumping will not exceed 0.5 m.

Pore w ater pressure difference (kPa) -100 0 100 200 300 400 0

2 Pore water pressure difference between 4 the pore water pressure 6 pre- and post-surcharge Pore water pressure 8 difference between the initial pore water pressure 10 and post surcharge

Depth(m) 12 Additional stress caused by surcharge 14 Pore water pressure difference between 16 the pre-vacuum unloading and pre-surcharge 18

Figure 17: Pore water pressure of different stages during the surcharge Vol. 18 [2013], Bund. G 1571

Pore water pressure difference (kPa) -100 -80 -60 -40 -20 0 20 40 0 2 4 6 Pore water pressure difference between the initial pore water 8 pressure and that pre vacuum unloading 10 Pore water pressuredifference between the initial pore water 12 pressure and that post vacuum unloading 14 Depth(m) 16 18 20 Figure 18: Pore water pressure pre- and post-vacuum unloading

Pore water pressure difference(kPa) 0 10203040506070 0 2 4

6 Pore water pressure difference between the 8 Pore water pressure one month after unloading and that before unloading 10 Pore water pressure difference between the 12 Pore water pressure 6 days after unloading Depth(m) and that before unloading 14 16 18 20 Figure 19: Pore water pressure during different time periods after unloading

Figure 13 demonstrates that on approximately the 50th day of vacuum pumping, the non- uniform change in the pore water pressure and the change in depth indicate that the directly effective range of vacuum pumping should reach at least 15 m below the surface. This result is observed because the pore water pressure at depths below 15 m has a uniform change; however, the soil layer above 15 m has a non-uniform change, indicating that the decrease in the pore water pressure can be partitioned into two parts: the decrease in the water level and the direct action of vacuum. The reinforcement effect of the soil mass beneath the PVDs is not significant. The surface settlement during the vacuum preloading stage and the settlement curves at the bottom of the PVDs show that the vacuum preloading has little reinforcement on the soil mass beneath the PVDs (Figure 16). Vol. 18 [2013], Bund. G 1572

(4) The analysis of the pore water pressure after the surcharge The embankment surcharge begins on the 65th day. The filling height of the embankment is approximately 7.65 m, and the surcharge load is 137.7 kPa; the surcharge curve is illustrated in Figure 3. When terminating the surcharge, the change in the pore water pressure at each depth below ground and the theoretical additional stress caused by the surcharge are shown in Figure 17. Figure 17 illustrates that a portion of the pore water pressure dissipates during the surcharge period. (5) The analysis of the pore water pressure during the combined preloading stage and post-unloading stage After the completion of the surcharge, the pore water pressure decreases. The dissipated pore water pressure before the unloading of the vacuum load is shown in Figure 18. On the 259th day, the vacuum load is unloaded; the pore water pressure before unloading and six days after unloading can be observed in Figure 19, which shows that the ranges of increase in the pore water pressure are almost identical for all depths, increasing by approximately 13.5 kPa on average, except at the 2 m depth. This result indicates that the water level rises approximately 1.3 m following the termination of vacuum pumping, which again confirms that the ground water level has a downward trend during the vacuum pumping. The pore water pressure one month after unloading shows a slight increase compared with the 6th day after unloading, which also indicates that the pore water pressure recovers gradually after unloading the vacuum load. As in the dissipative process, this recovery process also requires time. During the recovery process, the excess pore water pressure is also dissipating. Time(day) 0 100 200 300 400 500 600 0 -200 Left road shoulder (k23+597) Middle embankment(k23+597) -400 Right road shoulder (k23+597) -600 Left road shoulder (k23+655) Middle embankment(k23+655) -800 Right road shoulder (k23+655) -1000 -1200 -1400 Settlement(mm) -1600 -1800 -2000 Figure 20: Settlement curve of K23+597 and K23+655 Vol. 18 [2013], Bund. G 1573

Time(day) 0 100 200 300 400 500 600 0

Left road shoulder (k23+720) -500 Middle embankment (k23+720) Right roa shoulder (k23+720) Left road shoulder (k23+880) -1000 Middle embankment (k23+880) Right roa shoulder (k23+880)

-1500 Settlement(mm) -2000

-2500 Figure 21: Settlement curve of K23+720 and K23+880

The pore water pressure near the surface increases greatly, and it increases slightly and similarly deeper in the ground. The pore water pressure near the surface recovers quickly: first, it is close to the atmosphere such that the recovery of the hydrodynamic pressure is more rapid; second, its upper soil mass may be in the unsaturated region.

The characteristics of settlement and their comparison The influence of equivalent vacuum load Figures 20 and 21 are the surface settlement-time curves of the four sections of soft ground. As shown in these figures, with the application of the load, the soil settlement during the surcharge preloading and the combined vacuum and surcharge preloading are greater at first, relatively mild subsequently, and finally stable. To study the soil settlement characteristics during surcharge preloading and combined vacuum and surcharge preloading, this study analyzes the soil compression acting on the unit load; that is, the settlement is divided by the corresponding load at the moment. Figure 22 shows the settlement/load-time curves of the combined vacuum and surcharge preloading area considering only the upper surcharge load without considering the equivalent vacuum load (80 kPa). As shown in Figure 22, without considering the equivalent vacuum load, the settlements matched with the unit surcharge load in VS-B-1 and VS-B-2 are greater than those with the surcharge preloading, and it becomes particularly obvious during the filling period that the settlement before vacuum unloading is essentially stable. The corresponding settlement values of unit load for each section at this moment are listed in Table 3.

Vol. 18 [2013], Bund. G 1574

Table 3: Settlement/loading without the equivalent vacuum load Final settlement Surcharge load Pre last stage Final value Section (mm) (kPa) (mm/kPa) (mm/kPa) S-B-1 1790 152.4 11.4 11.4 S-B-2 1834 162.4 11.2 11.2 VS-B-1 2730 189.7 14.1 12.2 VS-B-2 2658 197.5 13.1 11.7

As shown in Figure 22 and Table 3, before the application of the last-stage load, the unit load settlement in the combined vacuum and surcharge preloading area is significantly greater than that in the surcharge preloading area and exceeds 16-26%. Therefore, we can draw the following conclusion: the vacuum load is different from the surcharge load in mechanism, which changes the boundary conditions of the underground pore water pressure, but compared with the surcharge preloading, the intervention of the vacuum load will increase the total soil settlement. However, when applying the last-stage load (the surcharge increment is 22 kPa for VS-B-1 and 32 kPa for VS-B-2) during the filling period, the unit load settlements in VS-B-1 and VS-B-2 decrease gradually. At the end of this stage of the surcharge, the vacuum load is unloaded immediately; therefore, the settlement of the unit load in VS-B-1 and VS-B-2 has not increased substantially when it becomes consistent with the surcharge pre-loading area, as shown in the second half of Figure22 and Table 3. If the 80 kPa positive load equivalent to the vacuum load is counted into the total load, then the settlement/load-time curves for each section are shown in Figure 23. The settlement/load values for each section when the vacuum is unloaded and those when the increasing surcharge is stable after vacuum unloading are listed in Table 4. As shown in Figure 23 and Table 4, the equivalent vacuum load and the real surcharge load are believed to be different: the vacuum load can increase the total soil settlement, whereas the settlement caused by the unit equivalent vacuum load is smaller than that caused by the real unit surcharge load.

Table 4: Settlement/loading with the equivalent vacuum load Final settlement Surcharge load Pre last stage Final value Section (mm) (kPa) (mm/kPa) (mm/kPa) S-B-1 1790 152.4 11.4 11.4 S-B-2 1834 162.4 11.2 11.2 VS-B-1 2730 189.7 9.4 12.2 VS-B-2 2658 197.5 8.9 11.7

Settlement uniformity The final settlements at the five monitoring points in each section are divided by the final settlement of the midpoint in this section to obtain the scale drawing of the section settlement after normalization. The ratio of the average settlement of the road shoulder and slope toe to the midpoint settlement in each section is listed in Table 5 and drawn in Figure 24, demonstrating Vol. 18 [2013], Bund. G 1575 that the settlement of the road shoulder in the combined vacuum and surcharge preloading area is notably similar to that in the surcharge preloading area; however, the settlement ratio of the slope toe is significantly greater than that in surcharge preloading area, exceeding 8%-18%. This shows that the settlement in the combined vacuum and surcharge preloading area is more uniform than that in the surcharge preloading area.

Table 5: Final average settlement of the shoulder and foot and the ratio of their settlement to the center Settlement ratio of the Settlement ratio of the Section shoulder to the center slope toe to the center S-B-1 88% 29% S-B-2 88% 26% VS-B-1 91% 37% VS-B-2 90% 44%

Time(day) 0 100 200 300 400 500 600 0

4 Surcharge load(S-B-2) Surcharge load(S-B-1) 6 Vacuum load(VS-B-2) Vacuum load(VS-B-1) 8

Settlement/Load(mm/kPa) 14 16 Figure 22: Settlement/loading vs. time without the equivalent vacuum load

Time(day) 00 100 200 300 400 500 600

4 Surcharge load(S-B-1) Surcharge load(S-B-2) 6 Vacuum load(VS-B-1) Vacuum load(VS-B-2) 8

Settlement/Load(mm/kPa) 14

16 Figure 23: Settlement/loading vs. time with the equivalent vacuum load Vol. 18 [2013], Bund. G 1576

Horizontal coordinates(m) -30 -20 -10 0 10 20 30 0

Surcharge load(S-B-2) 20 Surcharge load(S-B-1) Vacuum load(V-B-2) Vacuum load(V-B-1) 40

80 Settlement ratio after normalized(%) after ratio Settlement

100 Figure 24: Scale for comparison of the average settlement of the shoulder and foot to the settlement of the center

THE SETTLEMENT AND THE COMPUTATION OF THE DEGREE OF CONSOLIDATION

The computational theory According to the literature, the settlement computation is in accordance with the e-p curve layer-wise summation method. No further details will be given regarding the computational theory. The degree of consolidation is computed using the Hansbo solution(17)under equal strain axially symmetrical condition in the first-stage or multi-stage uniform speed load, where the consolidation time is T and the average degree of consolidation of the corresponding total load U t can be computed according to the following formula:

 n qi  t TTii1 UTTeeet ii1 (1)  p  i1  

U t ——Average degree of consolidation at the time of t; 

qi ——Loading rate of load grade i (kPa/d);  p ——The accumulative value of all load grades(kPa);

Ti1 、Ti ——starting and ending time of load grade i(d);  、  ——Parameters determined by drainage condition of the soil. For soil in which the compressed soil layers are not penetrated by vertical drain, the formula above should be used to compute the average degree of consolidation of the soil layer within the limit of the vertical drain and the average degree of consolidation of the compressed soil layers under the vertical drain. Vol. 18 [2013], Bund. G 1577

Indraratna et al. (10) suggested that if the longitudinal flow capacity of the vertical drain is below 400 m3 per year, the well resistance effect should be considered; Holtz et al. (18)suggested that if the longitudinal flow capacity of the vertical drain is above 150 m3 per year, the well resistance effect should be ignored. The longitudinal flow capacity of the PVDs in this paper is 40 cm3/s (the lateral pressure is 350 kPa), which is equal to 1261 m3 per year. That value is far beyond the suggested value of Holtz et al., and therefore the influence of well resistance is not taken into account in the computations.

Computation parameters The parameters used by the Geological Survey Report along the Zhong-Jiang Expressway for each soil layer related to this study are shown in Table 6.

Table 6: Calculation parameters 3 -3 2 -3 2 -1 Soil layer  0 (kN/m ) e0 Cv(×10 cm /s) Ch(×10 cm /s) av(MPa ) F

1. cultivated soil 18.0 1.056 8.60 31.00 0.350 4.176

2. shell/oyster 18.4 1.001 30.40 103.00 0.150 4.992

3. muddy clay 18.0 1.271 2.07 4.20 0.921 4.024

4. silt 18.2 1.280 2.90 8.60 0.621 3.844

5. silt 18.2 1.056 40.00 42.00 0.150 5.264

6. clay 18.5 1.172 3.50 9.20 0.560 4.149

7. muddy clay 17.8 1.623 1.35 4.90 1.570 3.843

8. pebble/gravel 18.6 0.901 386.00 403.00 0.100 14.513

The loading curves used for computation are illustrated in Figure 4, and the vacuum load is equivalent to the positive load. The computation settlement curves obtained after being multiplied by the correction coefficient (ms) are shown in Figure 25; the coincidence degree of the computed value and the measured value is high. The correction coefficients (ms) for each section are listed in Table 7. The settlement correction coefficient in the surcharge preloading area is larger than that in the combined vacuum and surcharge preloading area; this conclusion is consistent with the literature. Vol. 18 [2013], Bund. G 1578

Time(days) 0 100 200 300 400 500 600 0 -200 Calculated value(k23+597) -400 M easured value(k23+597) Calculated value(k23+655) -600 M easured value(k23+655) -800 Calculated value(k23+720) -1000 M easured value(k23+720) Calculated value(k23+880) -1200 M easured value(k23+880) -1400 -1600 Settlement(mm) -1800 -2000 -2200 -2400 Figure 25: Calculated and measured settlement curves of the center embankment in each section

Table 7: Settlement correction factor Calculated sections S-B-1 S-B-2 VS-B-1 VS-B-2

Correction factor ms 1.5 1.4 1.19 1. 10

CONCLUSION Based on the two field tests, this study examines the pore water pressure and the settlement characteristics of soil improved by combined vacuum and surcharge preloading and discusses and compares the methods for computing settlement and the consolidation degree of combined vacuum and surcharge preloading and surcharge preloading. The following conclusions can be drawn: 1. The non-uniform change in the underground pore water pressure and the change in the water depth in test A indicates that the directly effective range of vacuum pumping in this paper should reach 18 m below the surface and that in vacuum preloading, the range of the decline of the pore water pressure increases with decreasing distance below ground. The groundwater table level declines during the process of vacuum preloading, with the range of the decline less than 1.8 m. Additionally, the restoration of the negative pore water pressure after unloading takes time, as does the dispersion of the positive excess pore water pressure under surcharge preloading. 2. The combination of vacuum and surcharge load can both increase the rate of soil settlement and increase the total soil settlement; however, the settlement increment caused by the vacuum load will be less than that caused by the real surcharge load. The settlement of the road shoulder of the combined vacuum and surcharge preloading area and the surcharge preloading area are very similar, and the settlement ratio of the slope toe of the combined vacuum and surcharge preloading area exceeds the settlement ratio of the slope toe in the surcharge preloading area by 8%~18%; that is, the settlement caused by the application of combined vacuum and surcharge preloading is more uniform than that caused by the action of surcharge preloading. 3. The vacuum load can be equivalent to the positive load when settlement and the degree of consolidation of combined vacuum and surcharge preloading is calculated, and the settlement correction coefficient (ms) of combined vacuum and surcharge preloading is less than that of the surcharge preloading. As an equivalent load, the vacuum load can increase the total soil Vol. 18 [2013], Bund. G 1579 settlement during the construction period, and the internal shrinkage of the soil caused by the vacuum pumping causes the surcharge speed to be unrestricted; therefore, the full load preloading period can be extended relatively if surcharged as soon as possible. With the intervention of the vacuum load, the post-construction settlement can be effectively eliminated.

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